The bride and groom may say ‘I do,’ but the guests at a royal wedding must consider a long list of ‘you don’ts.’ Here are some of the rules for the May 19th event at St. George’s chapel.

DON’T Bare your shoulders Meghan Markle wore a strapless number at her first wedding in 2011. But not this time around: Bare shoulders are a no-no for royal brides — thus the enormous puffed sleeves on Princess Diana’s wedding gown. They’re also out for guests, so expect demure sleeves all around.

DON’T Get shoe-crazy

Meier also says sling-backs are out, as are open-toe shoes, wedges and stilettos. (No word on high-tops or thigh-high boots.)

DON’T Bring a sword — and mind your medals, will you? Though men may show up in military uniform, no swords are allowed. There’s also a reference to medals in the seven pages of “critical guidance” handed out to invitees and leaked to the press. The Daily Mail thinks this is a way to avoid a repeat of David Beckham’s faux pas at the 2011 royal wedding: The footballer wore his Order of the British Empire medal on the right side of his jacket rather than the left.

DON’T Arrive hatless … or madly hatted Anyone who’s watched Coronation Street knows British wedding equals lady-hats. And women invited to the royal wedding are expected to wear something on their heads, be it fascinator or chapeau. Just keep it simple: That mega-bow/pretzel/wiggly pink thing Princess Beatrice wore to the wedding of Prince William and Kate Middleton was still earning her criticism even after she raised about $130,000 for it at a charity auction: “One man emailed me telling me I had done more to bring down the monarchy than at any other time in history!” she recently told The Telegraph.

DON’T Show any leg, either An invite to St. George’s means “day dress with hat” — so cleavage is considered disrespectful and experts say hemlines shouldn’t be shorter than a finger-length above the knee or longer than mid-calf. As for that leftover bit of leg — it must be encased in a stocking — and only nude hues will do, according to Myka Meier, an etiquette expert who trained under a former member of the royal household.

DON’T Wear white, or anything too loud Even commoners know you don’t wear white to a wedding, lest you upstage the bride. At royal events, bold colours and patterns are also ill-advised. Men, meanwhile, are advised to wear “morning coats or lounge suits.” Sadly for men who chafe at dress-up, that does not mean sweats; the dress code is a waistcoat and tie, with a suit in grey, navy or black.

DON’T Go to the bathroom during the service Invitees have been warned of the chapel’s “very limited toilet facilities.” Among the rules: you can use that loo at the chapel from 9 to 11 a.m., but it will be “restricted” during the service.

DON’T Snap a selfie While hashtags are common at millennial weddings, royal guidelines explain that guests must abandon their phones and any picture-taking devices during the service. So much for #Harrkle4ever on Instagram — at least until they leave the church.

DON’T Touch the Queen Weddings can make people emotional, but don’t think of getting touchy-feely with Queen Elizabeth. Michelle Obama raised eyebrows in 2009 when she met the monarch and put her arm around her. Even though Elizabeth reciprocated, the press called it a scandal. So keep a stiff upper lip and your non-royal hands to yourself: the Queen must always make the first move, even if it’s just a handshake.